Most of my business comes from agencies who outsource such skills, maybe design agencies who don't have coding skills or facebook expertise. But I do gather work from the odd startup.
How I landed into contracting? I downloaded wordpress, setup a trivial website and optimised the site for adwords.
Adwords has been my biggest source of work from the start but its only a starting point. I have built up relationships with guys who will pass me work over and over. I use twitter and facebook to connect with the agencies to constantly update changes within facebook and the new work that I do.
Most of the agencies that I work with have multiple clients from different sectors and what works for one of my clients may work for a client of theirs completely different to whom they originally contacted me about.